The human immunodeficiency virus type 1 (HIV-1) reached Thailand in 1988 and the incidence has risen rapidly among high risk population, such as intravenous drug users and prostitutes. Recently, the seriousness of the epidemic is reflected in the 2% HIV-1 seropositivity rate among women attending antenatal clinics in Bangkok. Moreover, mother to child transmission of HIV was observed among 26% of the infants born to seropositive mothers. In Northern Thailand, where the rate of HIV-1 transmission is highest, the proportion of infected mothers and infants is expected to be greater. HIV-1 subtype E is the predominant viral genotype found among mothers and infants in Thailand. This viral subtype is genetically distinct from the predominant North American/European subtype B in the envelope, Gag, and protease regions of the viral gnome. This research proposes to characterize the biological and immunological properties of the viral subset involved in mother to child transmission. Further genetic diversity between paired mothers and infants will be evaluated to assess the impact of biological and immunological pressures in selecting the viral subset for transmission. The role of maternal antibodies in protection from viral transmission will be assessed with the autologous maternal and child viral isolates. Broadly reacting neutralizing HIV-1 monoclonal antibodies will be tested for their potential use as therapeutic agents to interrupt this transmission.